Visual methods for determining the size of particles have already been used in a very wide variety of ways. The light scattering plays a minor role in these methods. Here, the Rayleigh, Mie, Debye, Rayleigh-Gans, and Fraunhofer theories of scattering are used. While the Rayleigh theory of scattering can only be applied for particle sizes smaller than approximately 1/10 of the wavelength of the light which falls, and only implies one single scattering center, the Mie theory of scattering can be applied to particle sizes in the range of 0.1 to 10 times the measured wavelength. However, the Mie theory is highly complex and requires a powerful computer. The Debye, Rayleigh, Gans, and Fraunhofer theories of scattering are by contrast based on simple assumptions, and are accordingly less complex; however, they are usually only suitable for determining larger particles.
According to a first embodiment of known measuring devices for determining the size of particles in the nanoscopic range using light scattering, use is made of a light source of monochromatic light, i.e., laser light, and a plurality of detectors, which are attached at different angles to the sample bodies to be measured. A device of this type is described, e.g., in DE 696 00 969 T2. Here, a compound laser deflection instrument is used, with which individual scattering signals can be systematically generated. The method described in this document enables the measurement of particle size distributions via light scattering.
In a similar manner, in DE 197 24 228 A1, particle size distributions and concentrations of particles can be determined with the aid of electrically modulated light sources by detecting the scattered radiation under different scattering angles. For a fixed individual wavelength, the scattering angle is successively set by rotating a minor, and the respective scattered light portion is detected.
According to a second embodiment of known measuring devices to determine particle sizes or particle size distributions with the aid of a light source of monochromatic light, use is made not of individual, fixed position detectors, but of a field of a plurality of detectors. For example, according to the measuring device disclosed in DE 195 10 034 A1, a laser beam which is scattered on a dispersed particle sample is depicted by means of a field of photodetectors which are arranged at the burning level of the depiction device, and fed to an evaluation measuring device. The measuring device found is designed to enable the most compact and visually stable construction possible. Furthermore, the diameter of the measuring beam is designed to be variably adaptable to the demands of the respective measuring range, so that extended particle collectives can also be determined down to the finest particles with regard to their size distribution.
A third embodiment of known measuring devices for determining the particle size or particle size distribution using light scattering can be found in U.S. Pat. No. 6,137,572, in which by means of the dynamic light scattering which uses the double broadening of the scattered light against the narrow strip of the laser light which is beamed in, the sensitivity for the determination of the particle size is significantly increased again.
Finally, particle sizes or particle densities can also be determined according to a further embodiment of a measuring device known from the prior art by means of interference measurements, such as those disclosed, e.g., in DE 199 54 702 A1 and DE 195 25 847 A1. With the methods described in these documents, an interference image is generated with coherent light of a laser, following which the desired information on particle density and size can be estimated from the interference pattern received. Thus, for example, the device according to DE 195 25 847 A1 is equipped with a deflection unit which permits a change of direction of the illuminating laser beam in such a manner that during measurement, said laser beam is guided into an angle range around the particle to be measured.
The optical methods used to date to determine particle size or particle size distributions require expensive apparatus and also do not permit, or do not automatically permit, time-dispersed measurements.
It would therefore be desirable to be able to draw on measuring devices to determine the size of particles in the nanoscopic range using scattered radiation which are not encumbered by the disadvantages of the prior art.